1. Field of the Invention
This invention relates to the recording and reproduction of images. More particularly, the invention relates to the recording and reproduction of stereoscopic television or video images.
2. Description of Related Art
Stereoscopic or "3D" photographs and moving films have been known for many years. The photographs or films have been made by simultaneously photographing an object from two different aspects, through two lens systems separated horizontally or vertically from one another or by dividing a single lens system using mirrors, prisms etc. In the case of moving films, the usual technique has been to use different colour filters, for example red and green, with the two lens systems, or split lens system so that red and green images are formed simultaneously on each frame or alternate frames of the film. When the film is subsequently shown in the cinema or through a television transmission system, each member of the audience wears a pair of spectacles with complementarily coloured lenses, so that the two images on the screen are presented separately to the two eyes of the viewer, producing the illusion of a stereoscopic or three dimensional image. Disadvantageous of the known techniques are that they require specially built or specially adapted cameras which, because of the need for split or separate lens systems, are bulky and expensive, and that they produce a degraded or double image when the result is viewed without the appropriate spectacles.
Stereoscopic vision relies on the fact that each eye sees a different aspect of the same object. This stems not only from the different positions occupied by the two eyes, but from the fact that there is usually movement in the scene being viewed, or movement of the eyes themselves. A person's head is for a large part of the time in constant movement, as are the eyes within the head, as they constantly scan the scene they are looking at. This causes continuous change in the aspects of the scene being viewed by each eye, which is important in the perception by the brain of the "dimensionality" of objects in the scene. Any movement in the scene, even of a single moving object, will increase the stereoscopic effect, even if the eyes are kept stationary.
The present invention makes use of this effect in producing stereoscopic images in a television or video system.
Television or video systems use an electronic camera to generate, three electrical signals corresponding to the red, green and blue light from the scene being recorded. These signals are eventually reproduced in the television receiver to give rise to red, blue and green images which are in effect superimposed on the television screen to produce a full colour image. The images are produced in a sequence of "fields"; in a typical television system a field is produced by scanning approximately half the lines of the television picture, the remaining lines being scanned in the next field in a manner known as "interlacing", the two successive fields forming a complete frame of the picture.
There have been proposed systems for producing stereoscopic television or video images, in which the superimposed red, green and blue images on the television screen in each field correspond to images seen by the camera at times separated by predetermined intervals. Examples of such systems are shown in European patent Nos. 0 022 820B and 0 089 611B.
In these systems, the single colour images appearing on the television screen correspond to images of the scene taken at different times. Stereoscopic information about objects in the scene is effectively encoded in the disparity between the single colour images superimposed on the screen. However, this information will be different depending on the direction of movement of an object across the scene. For example, suppose an object is moving from left to right across the scene, and that two successive images are formed in red and cyan light and subsequently superimposed on the screen (a two colour system being considered for the sake of simplicity). From the point of view of the camera, the red image will correspond to the image of the object as seen from a point slightly to the right, as compared with the cyan image, the degree of misalignment or disparity depending on factors such as the speed of movement of the object and its distance from the camera. If the superimposed images are viewed through spectacles so that the red image is presented to the right eye and the cyan image to the left eye of the viewer, then the crossed horizontal disparity between the images will be equivalent to that caused by viewing an object suspended in front of the screen, simultaneously from points spaced to the right and left of a central position, as in normal binocular vision. If one now considers an object moving from right to left across the scene, the right and left eyes will again be presented with different images, but they will now correspond to the uncrossed disparate images of an object behind the screen. Since this is an improbable stimulus in normal binocular vision, it would be expected that no stereoscopic effect, or an incorrect stereoscopic effect, would be observed. Similarly, an object moving vertically in the scene would give rise to vertically disparate images, which would not be expected to give rise to a stereoscopic effect. If the camera undergoes translatory motion while viewing a three-dimensional scene, all objects in the scene, except those at optical infinity, will give rise to disparate images on each frame, the disparity varying appropriately with the distance of the objects from the camera. Camera motion in one direction (right to left) in the system described above) will generate disparities of correct sign, which would give rise to a strong stereoscopic sensation for the entire scene when the image is subsequently viewed. Camera motion in the opposite direction will reverse the sign of all disparities, which might be expected to produce a reversed stereoscopic sensation.
It has been found that reversed stereopsis rarely if ever occurs and the stereoscopic sensation is appropriate in direction whatever the motion of object or camera. It appears that the strong monocular cues (e.g. linear perspective, relative size, motion parallax) determine the depth impression of the picture and any horizontal disparities in the image can enhance but not override the depth sensation.